For example, I am terrible at Super Meat Boy, but just playing it has really improved how I play platformers and games that need faster imputs overall.
Well, I can tell you this. I grew up playing Mega Man. People say those games are hard, but I have them all memorized, so they’re all pretty easy for me.
Sometimes I play platformers that people consider hard, and I’m just disappointed by how mind-numbingly easy they are. Celeste is one example. I kept thinking, surely it must get harder. Maybe when I do the B sides. Surely there must be at least one part I struggle on. There never was. I never found anything hard about the game. The story was amazing, though.
So anyway, my answer is Mega Man. Not Mega Man X. Those games are amazing - quite possibly my favorite platformers of all time - but they’re too easy to fit into this category. The classic, 8-bit mega Man games from the NES (Mega Man 2 excluded. That one is also too easy).